Abstract. High-resolution hydrographic observations of temperature and salinity are
used to analyze the formation and distribution of isothermal depth
(ZT), mixed depth (ZM) and barrier layer thickness (BLT) in a
section of the southwestern Atlantic (0°30´ N–14°00´ S;
31°24´–41°48´ W), adjacent to the northeastern Brazilian
coast. Analyzed data consists of 279 CTD casts acquired during two cruises
under the Brazilian REVIZEE Program. One occurred in late austral winter
(August–October 1995) and another in austral summer (January–April 1997).
Oceanic observations are compared to numerical modeling results obtained
from the French Mercator-Coriolis Program. Results indicate that the
intrusion of subtropical Salinity Maximum Waters (SMW) is the major process
contributing to the seasonal barrier layer formation. These waters are
brought by the South Equatorial Current (SEC), from the subtropical region,
into the western tropical Atlantic boundary. During late austral winter
southeastern trade winds are more intense and ITCZ precipitations induce
lower surface salinity values near the equator. During this period a 5–90 m
thick BLT (median = 15 m) is observed and BLT > 30 m is restricted to
latitudes higher than 8° S, where the intrusion of salty waters between
8°–12.3° S creates shallow mixed layers over deep (ZT ≥ 90 m)
isothermal layers. During austral summer, shallow isothermal and mixed
layers prevail, when northeasterly winds are predominant and evaporation
overcomes precipitation, causing saltier waters at the surface/subsurface
layers. During that period observed BLT varies from 5 to 70 m and presents
thicker median value of 35 m, when comparing to the winter. Furthermore,
BLT ≥ 30 m is observed not only in the southernmost part of the study
area, as verified during late winter, but in the latitude range
2°–14° S, where near-surface salty waters are transported
westward by the SEC flow. These results indicate that the inclusion of
salinity dynamics and its variability are necessary for studying mixed and
barrier layer behaviors in the tropical Atlantic, where ocean-atmosphere
coupling is known to be stronger.